


Writing it Down

by yastaghr



Series: Sanster Continuum [4]
Category: Undertale (Video Game)
Genre: Aphrodisiacs, Creampie, Dirty Talk, Dom/sub Undertones, Dubious Science, Ecto-Body (Undertale), Ecto-Genitalia (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Foreplay, Gaster is married to Sans, Good W. D. Gaster, Hand Jobs, M/M, Mating Cycles/In Heat, Multiple Orgasms, Plot What Plot/Porn Without Plot, Rough Sex, Sanster, Suspension, Table Sex, Undertale Skeletons in Heat, Xenophilia, hands behind back
Language: English
Status: Completed
Published: 2020-09-24
Updated: 2020-09-24
Packaged: 2021-03-08 03:54:54
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,138
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26629375
Author URL: https://archiveofourown.org/users/yastaghr/pseuds/yastaghr
Summary: Gaster is messing around with science and accidentally induces an artificial heat. Luckily his husband, Sans, is willing to help.
Relationships: W. D. Gaster/Sans
Series: Sanster Continuum [4]
Series URL: https://archiveofourown.org/series/799875
Comments: 2
Kudos: 44





	Writing it Down

Gaster watched as his creation bubbled. It was currently a bright blue and fairly caustic, but he hoped that by adding the sulphide that it would stabilize the concoction into a more useful cleaning agent. Humans were obsessed with such weird things. He didn’t understand why they needed such a wide variety of cleaning agents, but the client was paying well, it was an interesting challenge, and he needed to do something to get back into the game after so long. 

This had seemed like a good plan at the time, but when he added the chemical to the solution it had… unanticipated results. For one thing, it blew up. That really shouldn’t happen in a casual lab like his, but magic always had an unpredictable factor that could make even the simplest of activities exciting. 

For another thing, it didn’t burn. Most cleaning products should not be applied to skin (or bone), but this one definitely wasn’t behaving like a cleaning product. When he wiped the blue goop off of his safety goggles he discovered that the entire room was covered in a thin layer of bubbling goop, including him. There wasn’t much that was actually touching him thanks to his insistence on proper safety gear. It only seemed to have gotten on the exposed bone of his skull. 

Finally, the mess it had made seemed to be giving off a strong smell. Gaster sniffed it experimentally. It seemed to invade his sinus cavities and fill his brain. He huffed, trying to get the smell out of him (really, what was he thinking?) but it was too late. The musky smell already seemed to have affected him, along with the rest of his physical exposure. He could feel his joints throbbing as magic gathered in the narrow ley lines there, the room suddenly felt too hot, and his mental process was interrupted with one thought: Sans. 

Where was Sans? Wasn’t he here a bit ago? Gaster urgently needed to find him to deal with this… ah, that was what it was. The solution he had created had induced an artificial heat. Well, he definitely needed his boyfriend for that, didn’t he? Besides, Sans was his supervisor. This was definitely the kind of accident you should report.

Gaster took one second to peel back the layer of magic that lined the room and made cleaning a breeze, dumped the mess in an appropriate way, and reapplied the layer of magic. He took another second to strip the PPE off of him and wash his face off in the sink. A third second was spent writing down a few quick notes in the log. The only difference between screwing around and science is writing it down! Then he opened the door that led to their break area and sang, “Sa~ans! I had an accident!" 

For some reason, that statement did not have the desired effect. Sans jumped up from where he had been making notes in his notebook. He must have gotten at least two seconds of hangtime. That was impressive. His face, however, was not. He looked scared to death. 

“gaster, are you alright? are you injured? oh, stars, i knew i shouldn’t have left you alone in there. there’s a reason why you used to insist on the buddy system when working with dangerous chemicals. where are you hurt? do i need to call an ambu-” Sans scarcely paused for breath throughout the entire speech.

Gaster interrupted him, “I’m fine, Sans. It appears as though the solution I created can induce artificial heats, however. I can already feel my magic gathering, and I’m sure I must smell delicious.”

Sans narrowed his eyes at him. “really. you’re sure you aren’t just horny and want to fuck? you know, like last time?”

“Ah, no,” Gaster could feel himself blushing, “I’ll admit that was… somewhat ill-advised. I should have just told you the truth. I apologize again. However, this time I really do think I’m in heat. You can smell for yourself and check my joints for comparison if you wish, and, if you so desire, I can teleport home and make use of one of our toys.”

Sans sniffed and shook his head. “no, i believe you. you smell like musk and black licorice. just let me finish up this observation and then i’ll help you. you might as well strip while I do that.”

Gaster shivered. He loved it when Sans got so commanding. It made his whole body shiver, not with cold or fever, but with desire. He striped out of his clothes faster than he ever had before. His body, which still retained some of its goopy properties even after all this time, was glowing faintly purple. His genitals had already formed a thin, tendril-like length on his pelvis. Gaster waved it experimentally. Yes, it still worked.

Sans finished scrawling that obnoxious excuse for handwriting of his and set the pencil aside. Then he looked up, a smile on his face and a twinkle in his eye. He stood up and sashayed over to Gaster, palming his dick almost immediately. “someone’s got a boner, heh heh heh.”

A smooth hand with a hole in its palm cupped the side of Sans’ face lovingly. “Oh, yes. Only for you, Sans.”

Sans’ grin spread. “now me, i don’t usually get aroused so easily, but your scent has got me going, doc. you in the mood for a pussy or an ass?”

Gaster sported a grin of his own. “Oh, whatever you’re in the mood for, Sans, you know that. Although… it has been a long time since I have gotten to cum inside your womb. Maybe you would oblige an old man and form one?”

“oh, you’re not that old, doc. you’ve still got the energy to fuck me senseless on a regular basis, right?” Sans joked as he wrapped his hand around Gaster’s length, pumping it teasingly in the way he knew would drive Gaster nuts. “i bet that you can cum more than once.”

Gaster countered, “I can’t do that if you make me waste a load outside of you. Please, pull down your pants and get on the table.”

“geez, buy a guy a drink first,” Sans said with a wink as he lowered his pants. His magic was summoned, the dark blue ecto body puffy and fat, just like Gaster liked it. His pussy was already glistening with precum. When Gaster ran his fingers through it it was already slick. 

“Hmm, it seems like I’m not the only one who has a boner. Or, at least, the pussy’s equivalent,” Gaster said with a smile. “Now all we need is a little bit of foreplay and we can start. Oh, and you still need to get on the table, Sans. Don’t think I haven’t noticed your lack of compliance. Get on the table on your stomach, please, with your legs dangling off the edge.” 

Sans scrambled to comply, just like Gaster knew he would. Being fucked from behind like that, as well as being unable to do anything about it one way or the other, were two of Sans’ favorite things when it came to sex. He loved it when Gaster took control. Just for the icing on the cake, Gaster said, “Oh, and put your arms behind your back with the forearms together. I need a good handhold if I’m to fuck you well.”

Oh, that got Sans going. He was absolutely dripping by the time he was in position. Gaster helped him get his hands into the right position with one hand while his other hand gathered up slick. Then, without any warning, he slipped a finger inside of Sans and held it still. Sans clenched around him, but Gaster had to fight this heat to not start moving immediately. He knew it would be more fun for both of them if he did this right. 

It wasn’t too long before Sans pleaded, “doc, you’ve gotta move! you’re driving us both nuts when all we both want is for you to stick your dick inside me!”

Gaster groaned. Dirty talk was one of his biggest kinks and Sans knew it. “Oh, you little sneak. I’m not going to fall for it, though. I want you to really enjoy this, and you’re no masochist. I need to make sure you’re prepared.”

“Then at least move your damned finger! or, better yet, put in a second one and get to work! i can handle it. please!” Sans begged.

“Impatient,” Gaster tutted, “but you have a point.” He inserted a second finger and began working them. The wet sounds his fingers made as they scissored Sans open were music to Gaster’s non-existent ears. He listened to them, dick twitching, until he was sure that Sans could take him. Then he pulled them out to the tune of a groan from Sans. “Easy, Sans. I’m sure you’ll like what I’m about to do to you a hell of a lot more than just my fingers.”

Sans whined. “oh, please doc, slide that sexy tentacle of yours inside me. fuck, i can already picture it. please!”

Gaster obliged him, thrusting his whole dick into Sans in one go. Sans’ pussy spasmed around him, but Gaster was done waiting. He gripped Sans’ arms with both hands and set a brutal pace. Sans groaned and moaned his name, but Gaster was beyond words, at least for now. This artificial heat had built up inside of him to the point where he needed to get off before he could use words. 

Sans didn’t seem to mind. “fuck, doc~ you feel so good inside me. i can feel you all the way inside me. your length is perfect for me, and this pace- this pace is wonderful! mmh, yes.”

His words only drove Gaster on faster. Soon the friction built to the point where Gaster felt he was about to cum. He buried himself up to the hilt inside of Sans and let his orgasm rock over him. Purple seed spurted inside of Sans’ womb. Gaster saw it when he flipped Sans over onto his back. The little pool of purple got him going again. He snapped his hips back and resumed his rough pace. Sans’ moans grew until they were a cacophony of sound. 

“Oh, Sans, you have no idea how good this feels. You like this, huh? You like my twisty little dick cavorting inside your cunt? You like it when I make the table squeak across the linoleum with every thrust? You like being helpless in the face of my need to fuck?” Gaster asked lasciviously.

Sans groaned. “fu~ck! doc, i love it! i love your twisty little tentacle in my big, fat hole. I love the way it feels against my g-spot. oh, fuck, i’m getting close!”

Gaster moaned and sped up his thrusts. “Oh, please, squeeze your pussy tight around me!”

His husband obliged, cumming with a cry and clamping down on his dick. Gaster, spurred on by his induced heat, pounded him right through his orgasm and out the other side. Sans’ cries grew desperate as another orgasm built inside him. Gaster dragged at his hips, slamming into Sans’ pussy and stroking his good spots. 

This time, when Sans came, it was with a wail. He went on fucking Sans right through it, his own orgasm just a few more strokes away. Sans squirmed and Gaster tried to pull out. He didn’t want to make his husband uncomfortable. However, Sans grabbed his arms and dragged himself back onto Gaster’s tentacle until it was buried to the hilt. “keep it up, g. i’ve got another one in me, and i’m sure you do. you still smell worse than my sock collection.”

That comment made Gaster growl and turn up the intensity from 9 to 14. “Oh, really? You think I smell bad? Or do you just want another load of cum in your chubby, tight pussy?”

Sans didn’t answer him. He was too busy trying to breathe. His tongue was bouncing up and down along with his ecto fat folds. His eye lights had gone out. Gaster grinned. He loved making Sans lose it like that. He kept on fucking Sans hard until his own orgasm overcame him. He buried himself up to the hilt again, and this time he got to see his own dark purple magic spurting into Sans’ womb. He grinned. Oh, how his possessive side loved to see that. It was so… satisfying.

Gaster waited a bit to see if the heat was satisfied yet or not. Partially seemed to be the answer. It was satiated enough to let him take a break and take care of his husband. Gaster scooped Sans up in his arms and headed for the bathroom.

**Author's Note:**

> You can find me on:
> 
> Pillowfort - https://www.pillowfort.social/Yastaghr  
> Tumblr - https://yastaghr.tumblr.com  
> Twitter - https://twitter.com/yastaghr * Most active


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